Angle Finder

Need the angle of a staircase, a roof, a ramp or a corner that isn’t square? Lay your phone on the surface and this angle finder reads the tilt from its motion sensors, with a zero button for relative angles and a hold button to freeze the reading. No phone handy? Enter rise and run, or three measured lengths, and it calculates the angle — plus mitre angles for frames and polygons.

Four ways to find an angle

Using your phone as an angle finder

1. Press “Start sensor” — some phones ask for permission to use motion sensors.
2. Lay the phone’s long edge along the surface you want to measure.
3. Read the angle from level, or press Zero on a reference surface and then measure relative to it.
4. Press Hold to freeze the reading before you pick the phone up.
5. Check the reading on a known flat surface first; phone cases with bumps can add a degree or two.

Phone sensors are typically accurate to about a degree — fine for DIY, not for precision engineering. Laptops and desktop computers usually have no tilt sensor.

Finding an angle from rise and run

Place a level against the surface, measure a horizontal distance along it (the run) and the vertical distance from the level to the surface (the rise). The angle is arctan(rise ÷ run). A staircase with a 7-inch rise and an 11-inch run slopes at about 32.5°, a 63.6% grade or 7.6 in 12.

Checking a corner with three measurements

Measure an equal or known distance along each edge from the corner, mark both points, then measure straight across between the marks. The law of cosines turns the three lengths into the corner angle: cos A = (a² + b² − c²) ÷ 2ab. With 30 and 40 along the edges and exactly 50 across, the corner is exactly 90° — the classic 3-4-5 check. If the diagonal is 51, the corner is open by about 2.4°.

Mitre angles for frames and polygons

For a regular polygon with n sides, set the mitre saw to 180 ÷ n degrees. For corners that aren’t quite square, measure the actual angle with the three-sides method and cut each piece at half of it.

Worked example: a wall that isn’t square

Fitting skirting board into a corner, a carpenter measures 30 inches along each wall and 43.5 inches across. The three-sides method gives a corner angle of about 93.0°, so each piece should be cut to meet at 46.5° rather than 45° — a mitre saw setting of about 43.5°. Cutting at the standard 45° would leave a visible gap at the front of the joint.

Calibrating your phone

Place the phone on a surface you know is level — checked with a spirit level — and note the reading. If it isn’t zero, press Zero there, and later readings will be relative to true level. Remove bulky cases, and measure along the same edge of the phone each time. For long surfaces, rest a straight board on the surface and put the phone on the board to average out bumps.

Bevel and compound angles

Some cuts tilt the blade as well as swinging it — crown moulding and angled boxes need compound angles. Use the angle finder to measure each surface separately, and follow your saw’s compound mitre chart for the combined settings.

Degrees, percent and pitch

The same slope can be written in several ways. Degrees measure the angle from horizontal; percent grade is rise divided by run times 100 (so 45° is 100%); and pitch, used for roofs in North America, is inches of rise per 12 inches of run. The tool shows all three so you can match whatever your plan, code or product uses.

Common angles around the home

Building codes vary — check local rules for ramps, stairs and roofs.

Privacy

Sensor readings and measurements stay in your browser; nothing is uploaded.